Frequent use is made of an IGBT (insulated gate bipolar transistor), which is a switching element, as a motor controlling element. When shutting down an IGBT by sharply reducing a gate signal of the IGBT, the main current of the IGBT decreases sharply, and an overvoltage is applied to the IGBT due to inductance of a main circuit. When an overvoltage exceeding the withstand voltage of the IGBT is applied, the IGBT is destroyed.
FIG. 10 is a block diagram showing a system when a motor is driven by an IGBT. When the IGBT operates abnormally (overcurrent, overheat, or the like), or when an unshown external alarm signal is input, it is required that this is detected, and that the output of the drive IC is shut down, thus stopping the operation of the IGBT. At this time, as the IGBT is destroyed as heretofore described when an abrupt signal shutdown is executed, it is required that a shutdown mode wherein the shutdown signal is gentle (called a soft shutdown) is adopted.
FIG. 11 is a block diagram showing a configuration of a drive IC 500. A description will be given of each terminal formed in the drive IC 500. In FIG. 11, IN is an input terminal that receives an operation signal at a time of normal operation, and OUT is an output terminal that outputs a signal for driving a controlling element (IGBT). OC1 and OC2 are terminals that detect an IGBT overcurrent, while OH1 and OH2 are terminals that detect an IGBT overheat. GV is a terminal for monitoring the gate voltage of the IGBT, REF is a reference voltage terminal of each kind of circuit, and VOH is a terminal that determines a reference voltage when an overheat is detected. Also, AE is a terminal that outputs (or inputs) an alarm signal.
FIG. 12 is a diagram showing a main portion circuit diagram of a heretofore known drive IC and a block diagram of a system including the drive IC, a main driver, an IGBT, and a motor. Each terminal, such as VCC, IN, OUT, and PGND, each MOSFET 55, 56, and 57, a logic circuit 53, and an alarm signal processing circuit 54 are formed in a semiconductor substrate 501. Also, the p-MOSFET 57 configures a shutdown circuit 52.
An operation signal is input into the input terminal IN at a time of normal operation, and the operation signal controls a signal output from the output terminal OUT by alternately turning the p-MOSFET (a p-channel type MOSFET) 55 and n-MOSFET (an n-channel type MOSFET) 56 on and off via the logic circuit 53. When the p-MOSFET 55 is turned on, an on signal (H signal) is applied from the output terminal OUT to the gate of the IGBT 61 via the main driver 69. When the p-MOSFET 55 is turned off, and the n-MOSFET 56 is turned on, an off signal (L signal) is applied from the output terminal OUT to the gate of the IGBT 61 via the main driver 69.
At a time of abnormal operation, irrespective of the signal from an unshown control circuit in the logic circuit 53, the p-MOSFET 55 and n-MOSFET 56 are both turned off via the alarm signal processing circuit 54 and logic circuit 53, and an on signal is applied from the logic circuit 53 to the gate of the p-MOSFET 57 configuring the shutdown circuit 52. By so doing, the p-MOSFET 57 is turned on, and the gate voltage of the IGBT 61 is lowered to the ground potential via the output terminal OUT and main driver 69. At this time, a charge accumulated at the gate of the IGBT 61 is drawn along a route from the main driver 69 through the output terminal OUT and p-MOSFET 57 to the ground terminal PGND (to be exact, the p-MOSFET 57 draws current from the input portion of the main driver 69, and the main driver 69 amplifies the action of the input portion, thus drawing the charge accumulated at the gate of the IGBT 61. In this case, the main driver functions as one kind of current amplifier). As the shutdown circuit 52, that is, the p-MOSFET 57, is designed so as to turn on gently (has soft shutdown characteristics), the charge accumulated at the gate of the IGBT 61 is drawn gently, and the IGBT 61 shuts down softly.
FIG. 13 is a diagram showing voltage-current characteristics of the output terminal OUT at a time of abnormal operation. The voltage on the horizontal axis, being the voltage of the output terminal OUT, is the voltage between the source and drain of the p-MOSFET 57. Also, the vertical axis, being the current of the output terminal OUT, is the source current the p-MOSFET 57. The voltage-current characteristics of the output terminal OUT are such that, in accordance with the output characteristics of the shutdown circuit 52, that is, the p-MOSFET 57, an increase in current in a region of low voltage is gentle, and the rise in current becomes somewhat steep when the voltage is moderately high (depending on the characteristics of the diode connected p-MOSFET 57). As the current increases, and the resistance between the source and drain of the p-MOSFET 57 becomes dominant, in a region in which the voltage is higher still, the p-MOSFET 57 is designed so as to have soft shutdown characteristics such that the current becomes gentle again.
In a system mounted in a vehicle such as an automobile, a battery voltage (VB), which is a power source, is normally 10 to 20V. The power source terminal (VCC terminal) of the drive IC 500 is connected to VB, meaning that, when using a source follower type of p-channel type MOSFET like the p-MOSFET 57, a voltage of 0V to VB (the battery voltage) is necessary as a gate input signal of the p-MOSFET 57.
The heretofore described voltage-current characteristics of the output terminal OUT at a time of abnormal operation are primarily determined by the design of the p-MOSFET 57 configuring the shutdown circuit 52. Herein, an example is given of a case wherein the p-MOSFET 57 is designed in such a way that the increase in current in a low voltage region is small, the increase in current becomes larger as the voltage is raised, and the increase in current becomes gentle again when the voltage becomes higher still. The dotted line represents voltage-current characteristics when the shutdown circuit 52 is configured of an n-MOSFET instead of a p-MOSFET. Characteristics are shown such that the current increases sharply in a low voltage region, and the current is saturated as the voltage rises. Soft shutdown characteristics cannot be obtained with these n-MOSFET voltage-current characteristics. However, as there is an advantage in that the drive voltage is low, and the like, there are cases in which an n-MOSFET is used in a system.
FIG. 14 is a diagram of output terminal OUT voltage waveforms at a time of abnormal operation. In the waveform diagram in the case of the p-MOSFET 57 shown by the solid line, the fall of the voltage is gentle, while in the waveform diagram in the case of the n-MOSFET shown by the dotted line, the fall of the voltage is abrupt. Also, when the voltage of the output terminal OUT becomes sufficiently low, reaching a sink switching voltage, the n-MOSFET 56 is turned on, and the voltage of the output terminal OUT becomes the potential of the ground terminal PGND. The sink switching voltage is a voltage that switches the on condition from the p-MOSFET 55 or p-MOSFET 57 (source side) to the n-MOSFET 56 (sink side).
FIG. 15 is a main portion configuration diagram of a heretofore known system. An FWD 66 (freewheel diode) is connected to the IGBT 61, and a current detecting resistor 67 is connected to an IGBT 61 current sensing (detecting) emitter 64. Although not shown, there is also a method of detecting current by detecting the voltage between the emitter and collector of the IGBT 61. Also, a p-n diode 68 for temperature sensing is connected adjacent to the IGBT 61. The drive IC 500 includes the power source terminal VCC, the output terminal OUT, the gate voltage monitoring terminal GV, the overcurrent monitoring terminal OC, the overheat monitoring terminal OH, a ground terminal GND, the alarm terminal AE, and the input terminal IN. The output terminal OUT is connected to the input of the main driver 69, and the output of the main driver 69 is connected to the gate of the IGBT 61. The gate of the IGBT 61 is connected to the gate voltage monitoring terminal GV. The current sensing emitter 64 is connected to the current detecting resistor 67, and the high potential side of the resistor 67 is connected to the overcurrent terminal OC. The anode of the temperature sensing p-n diode 68 is connected to the overheat monitoring terminal OH, and the cathode is connected to the ground terminal GND. The battery voltage VB is input into the power source terminal VCC, an input signal is input into the input terminal IN, and an alarm input-output signal is input into and output from the alarm terminal AE.
Depending on the system, there are cases in which the output terminal OUT of the drive IC 500 and the gate of the driven IGBT 61 are directly connected, without passing through the main driver 69. Also, temperature sensing is carried out by the forward voltage of the p-n diode 68. Also, an operation signal from the exterior is input into the input terminal IN.
FIG. 16 to FIG. 19 are timing chart diagrams of various kinds of operation of the drive IC 500.
(1) Regular On-off Operation (Normal Operation: FIG. 16)
When the input terminal IN signal changes from an H level (OFF) to an L level (ON), the output terminal OUT signal changes from an L level (OFF) to an H level (ON), and the IGBT is turned on. At this time, the alarm terminal AE is at an H level (non-alarm condition).
(2) Power Source Voltage (VCC) Drop, Overcurrent, and Overheat Protection Operations (FIG. 16, FIG. 17, and FIG. 18)
On any kind of protection signal reaching a set voltage when the input terminal IN signal is at the L level (ON), the output terminal OUT signal changes from the H level (ON) to the L level (OFF). In the event of an abrupt change to the L level (OFF) when the output terminal OUT signal changes from the H level (ON) to the L level (OFF), the current flowing through the IGBT 61 also changes sharply. When this happens, an excessive surge voltage is generated by the inductance (floating inductance, or the like) of an external circuit, and there is a possibility of the IGBT 61 being destroyed due to overvoltage. Because of this, the output terminal OUT signal is changed to the L level (OFF) softly. The voltage of the output terminal OUT is monitored by the gate voltage monitoring terminal GV, and on the voltage (sink switching voltage) of the output terminal OUT dropping until a large current ceases to flow through the IGBT 61, the voltage of the OUT terminal is changed absolutely to the L level (GND) at that point, and the IGBT is turned off.
Also, on any kind of protection signal reaching the set voltage, the alarm terminal AE signal is changed from the H level (non-alarm condition) to an L level (alarm condition). The L level is maintained for a set certain period. Then, after the certain period, the alarm terminal AE signal changes from the L level (alarm condition) to the H level (non-alarm condition). When there is a protection detection condition, and the input terminal signal is at the L level (ON), after the certain period, the alarm condition continues until these conditions are eliminated.
Although not shown, the two functions of power source voltage drop and overheat protection are such that, on either kind of protection signal reaching the set voltage even when the input terminal IN signal is at the H level (OFF), the alarm terminal AE signal is changed from the H level (non-alarm condition) to the L level (alarm condition), and the L level is maintained for the set certain period. Even in the event that the input terminal IN signal changes to the L level (ON) condition within the certain time for which the L level is maintained, the output terminal OUT signal does not change to the H level (ON). When there is a protection detection condition, and the input terminal signal is at the L level (ON), after the certain period, the alarm condition continues until these conditions are eliminated.
(3) Operation when External Alarm is Input (FIG. 19)
When an L level (alarm condition) signal is input into the alarm terminal AE from the exterior, the output terminal OUT signal changes to a soft shutdown (OFF) signal. When an H level (non-alarm condition) signal is input into the alarm terminal AE from the exterior, the output terminal OUT signal changes back to an H level (ON) signal.
Also, soft shutdown technologies being shown in Patent Documents 1 to 5, technologies whereby the gate voltage is lowered in accordance with the output of a CR time constant circuit are disclosed in Patent Documents 1 to 3. Technologies whereby the gate voltage is lowered by changing the voltage dividing value of the resistor are disclosed in Patent Documents 4 and 5.